The development of an organism may be thought of as a set of interfacing developmental pathways each of which is under the control of a specific battery of genes. The bithorax gene complex in Drosophila appears to be such a battery. Analysis of that complex has already provided evidence that specific genetic units within the cluster interact to determine whether certain body segments achieve a mesothoracic, metathoracic or abdominal level of development. It is proposed to use mutant deficiencies, duplications and other rearrangements of the complex as probes to understand the nature of the development defects which the mutants produce and in turn to infer the function of the corresponding wild-type genes of the complex during normal development. It is proposed (in a collaborative study) to use a homozygous deletion (Df-P9) for all of the genes of the bithorax complex in a procedure to isolate and characterize DNA sequence complexity of the complex. SEM and histological techniques will be used to investigate the effects of deficiencies and regulatory-like mutants at different times in development employing somatic mosaics generated from unstable ring chromosomes and from X-ray induced somatic crossing over. Finally, existing regulatory-like mutants of the complex will be studied by varying the number of doses of these mutants in the genome as well as by creating somatic mosaics for such mutants.